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Requirements for Total Internal Reflection (TIR):
Two requirements must be met for total internal reflection to occur. First, light must be in the more dense medium heading toward the boundary with the least dense medium. Second, light must be approaching the boundary with an angle of incidence that is greater than the value of the so-called critical angle.
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Each diagram shows two media (air and water), the boundary which separates them (solid black line) and the incident ray (red line with arrowhead). For total internal reflection to take place, the incident ray must be in the more dense medium (water) at an angle of incidence that is greater than the critical angle. If uncertain, measure the angle of incidence (between the incident ray and the normal line) to insure that it is greater than the stated critical angle.
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Definition of Critical Angle:
The critical angle is the largest angle of incidence for which refraction can occur for a particular boundary. When light is incident toward a boundary with a less dense medium at the critical angle, light refracts into the new medium with an angle of refraction of 90 degrees.
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